This invention relates to the preparation of flexible polyurethane foam, and more particularly to the preparation of such foam using cell opening or softening additives.
In preparing many types of polyurethane foam, particularly the flexible and semi-flexible types, it is necessary that the walls of the individual cells in the foam be ruptured at some point during the foam production. Unless the cells are opened in this manner, the gasses trapped within the cells contract as the foam cools, creating a vacuum in the foam and causing the foam to shrink or collapse.
In many instances, the cell walls will not spontaneously rupture as the foam is made. Accordingly, it is often necessary to promote cell wall rupture either through mechanical means such as crushing, or with the use of certain types of cell opening additives. Certain types of copolymer polyols, particularly those which are prepared by the in situ polymerization of ethylenically unsaturated monomers in a continuous polyol phase, are effective cell openers. Other types of cell openers include certain polyolefins, particularly polybutene and polybutadiene rubbers, and polyethers of up to about 3500 molecular weight which contain a high proportion (usually 50 percent or higher) of oxyethylene units or units derived from butylene oxide. The polyether types are most commonly used when the foam is based on a poly(propylene oxide) polyol.
These cell openers all have certain drawbacks which limit their use or effectiveness. The copolymer polyols must be used in relatively high amounts, so that the copolymer polyol itself becomes a substantial if not major component of the active hydrogen-containing composition which is used to make the foam. When other types of polyols are desired, the copolymer polyol cannot be used in effective amounts. Many of the polyolefins and polybutadienes are less effective than desired and have the additional drawback of bleeding out of the foam, giving it an oily feel and making it more difficult to paint or glue to other materials. They are also incompatible with the other materials used in making the foam, which prevents them from being formulated into the polyol or polyisocyanate blends used in making the foam. Thus, these cell openers must be metered separately into the foam formulation simultaneously with or immediately prior to the start of the foaming reaction. The polyethers are likewise less effective than desired and are often incompatible with the active hydrogen-containing materials used.
Certain types of starting materials provide foams in which the cells are particularly difficult to open. Among these are the so-called "PIPA" polyols, which are dispersions of polyurea or polyurethane particles in a continuous polyol phase. Although foam made from these materials otherwise have many desirable properties, it is often very difficult to open their cells. Often, the use of particular surfactants and other raw materials compounds the problem.
Often, particularly when using an MDI-based polyisocyanate, the foam is stiffer and more boardy than desired. In those instances, it is often desirable to provide a means by which the foam is made softer.
Accordingly, it would be desirable to provide a cell opener which is effective at low levels and which is substantially compatible with the foam itself and with at least some of the components used in making the foam. It is further desirable to provide a cell opener which is effective in making certain foam in which the cells are particularly difficult to open, such as foam based on a PIPA polyol. In addition, it would be desirable to provide a means whereby a softer foam is prepared, particularly a foam prepared from an MDI-based polyisocyanate.